Ecdysone is a steroid hormone used by Drosophila to time developmental events, especially larval molts and pupariation. Bursts of ecdysone initiate cascades of gene activation. Many of the early genes in these cascades code for transcription factors that mediate the effects of ecdysone. While many genes become active during these cascades, others that were active before are suppressed. A simple genetic screen for mutants of these transcription factors would be to join a gene for a fluorescent probe to a promoter that is down regulated by ecdysone, FTZ-F1 for example. In this assay wild type larvae would look normal, but a mutant that fails to suppress the reporter gene would fluoresce green. While many developmental mutants are recessive lethals, the quantitative requirements for ecdysone by many genes and the interactions among transcription factors suggest that subtle dominant effects may be evident. It is possible to test the overall effectiveness of the assay by making use of existing mutants in known ecdysone responsive transcription factors, Usp for example. In addition, the genetic requirements for ecdysone synthesis are being investigated. The ecdysoneless gene, ecd, has been deficiency mapped to a region of 12 kb, and a number of cDNAs that hybridize to this fragment have been identified.